Si Spacer Research Articles

Optical Properties of Multilayered Staggered SiGe Nanodots Depending on Si Spacer Growth Temperature

The optical properties of the multilayered staggered SiGe nanodots (NDs) embedded in the Si spacers fabricated at varying Si growth temperatures are clarified in conjunction with the effect of strain by Photoluminescence (PL) and Raman spectroscopy. We found that compressive strain is induced in the SiGe NDs, with higher growth temperatures of the Si spacer resulting in stronger compressive strain. PL spectra indicate that the higher growth temperature leads to a wider bandgap. This transition energy behavior is caused by not only the strain in the SiGe NDs but also the Ge segregation. Furthermore, luminescence from the SiGe NDs was observed along with luminescence originated at the Si/SiGe interface in the SiGe NDs with a small dot size. This phenomenon was caused by the small energy difference of the conduction-band minima between the SiGe NDs and the tensile-strained Si spacers and the small volume of the tensile-strained Si spacers.

An Assessment of the Lateral Selective Etching, with HCl, of High Versus Low Ge Content SiGe Layers

We have evaluated the feasibility of selectively etching, with gaseous HCl, high versus low Ge content SiGe layers. To that end, we have grown {20 to 25 nm thick Si / ~ 12-15 nm thick Si0.8Ge0.2 or Si0.75Ge0.25 / ~ 11-14 nm thick Si0.5Ge0.5} multilayers on SOI substrates. After a low temperature capping with SiN, such stacks were patterned into regular arrays of square mesas or lines with a stop on the buried oxide, to laterally etch the layers of interest. We first determined, at 500°C, 600 Torr, the (001) blanket HCl etch rates of SiGe 30% and 40%: 1.4 and 5.3 nm min.-1. We then switched over to patterned wafers. The lateral selective etching of Si0.5Ge0.5 versus Si0.8Ge0.2 (and Si) was feasible and the following features highlighted: (i) Si0.5Ge0.5 lateral etch rates along the <110> directions were much smaller than in the vertical [001] direction and (ii) because of micro-loading, lateral etch rates were definitely higher for squares than for lines (0.9-1.1 nm min.-1 for lines, to be compared with 1.7–2.1 nm min.-1 for squares). Meanwhile, the lateral selective etching of Si0.5Ge0.5 versus Si0.75Ge0.25 was far from being perfect, with much higher Si0.5Ge0.5 etch rates and a significant Si0.75Ge0.25 recessing at tunnel entrances. Si0.75Ge0.25 floors and ceilings were otherwise far more etched and thus tilted than Si0.8Ge0.2 ones. Inserting very thin Si spacers between Si0.5Ge0.5 cores and Si0.75Ge0.25 claddings partly solved those issues, with no recessing and lesser etchings at the entry points of tunnels.

Evaluation of Thermal Transport Characteristics of Three-Dimensional Self-Ordered Multilayered Silicon-Germanium Nanodots

1. Background and purpose Silicon-germanium (SiGe) alloys have much lower thermal conductivity than Si and Ge single crystals and are expected to be candidates for next-generation thermoelectric device materials of internet of things society. It is important to achieve both high carrier mobility and low thermal conductivity on the basis of the dimensionless figure of merit [1]. There are several studies on the thermoelectric properties of SiGe with low-dimensional structures such as superlattice and nanodots [2,3] to dramatically decrease thermal conductivity. However, the relationship between the nanoscale thermal properties and the phonon scattering mechanism of self-ordered multilayer SiGe nanodots with high uniformity grown by Stranski-Krastanov [4] mode is unclear. In this study, we demonstrated the thermal transport characteristics of the self-ordered multilayer SiGe nanodots. 2. Experiments Si/SiGe superlattice and nanodots samples were fabricated by reduced pressure chemical vapor deposition. We set to 20 cycles of Si0.65Ge0.35 / Si superlattice, the staggered Si0.65Ge0.35 nanodots (alternately stacked) and the dot-on-dot Si0.65Ge0.35 nanodots (vertically stacked) by changing growth temperature and precursors for Si spacer growth [4], as shown in Fig. 1. Thermal properties were evaluated by the Time Domain Thermoreflectance (TDTR) method. The strain states and Ge fraction were evaluated by Raman spectroscopy. The focal length and wavenumber resolution of the Raman spectrometer were 2,000 mm and 0.1 cm-1, respectively. The excitation light source was a green laser (wavelength: 532 nm). We obtained the Raman spectra of Si-Si, Si-Ge, and Ge-Ge vibrational modes derived from the SiGe regions. 3. Results and discussion Figure 2 shows the thermal conductivity for the SiGe region of the superlattice and nanodots (dot-on-dot and staggered structures) obtained by TDTR method. We found that the SiGe nanodots with the dot-on-dot structure have the highest thermal conductivity, while the SiGe nanodots with the staggered structure and SiGe superlattice have almost the same thermal conductivity. We consider that the heat flow easily penetrates from the top (surface) to the bottom (Si substrate interface) in the dot-on-dot structures. On the other hand, it is possible that the staggered SiGe nanodots can play a sufficient role as a phonon scatterer compared to the dot-on-dot structures since the superlattice structure and the staggered SiGe nanodots have almost the same thermal properties. In conclusion, we demonstrated the possibility of controlling phonon scattering by changing the arrangement of the self-ordered multilayer SiGe nanodots. Acknowledgement This work was supported by JSPS KAKENHI Grant Number 21K14201, Japan.

Enhancing performance of silicon/graphene composites by transition lattice interfaces constructed using plasma

The silicon-based (Si) anodes have not widely used due to their low conductivity and severe irreversible volume changes until now. Graphene (G) is considered a promising material for inhibiting volume expansion and enhancing conductivity of Si, a large amount of work has been exploring effective composite of graphene and silicon. A new SiG composite structure (PB-SiG-1) with transition lattice interface is provided by N plasma assisted technique in this work. The phase interface undergoes lattice changes on the atomic scale with silicon, oxygen, and carbon bonding in new chemical bonds (Si-O-C). More importantly, the lattice spacing of Si (111) increases from 0.31 nm to 0.325 nm due to the insertion of carbon or oxygen atom into the Si body, which promotes the diffusion kinetics of lithium ions. This PB-SiG-1 exhibits better electrochemical performance with reversible specific capacity of 2013.9 mAh g−1 after 100 cycles at 0.2 A g−1, high capacity retention of 98.53% and coulombic efficiency of 99.4%. The multiple advanced in situ methods reveal that the PB-SiG-1 not only has the ability to quickly transfer ions, electrons, and form stable SEI, but also has a low irreversible volume expansion of 33.1 %. This proves that the transition lattice interface endows PB-SiG-1 with isotropy characteristic of lithium diffusion, leading to dynamically stable electrodes during cycles. This work proposes a Si-C composite structure with transition lattice interface, which opens a new insight for volume expansion and lithium transmission in Si-C composite.

High-Rate Performance of a Designed Si Nanoparticle–Graphite Nanosheet Composite as the Anode for Lithium-Ion Batteries

A silicon nanoparticle–graphite nanosheet composite was prepared via a facile ball milling process for use as the anode for high-rate lithium-ion batteries. The size effect of Si nanoparticles on the structure and on the lithium-ion battery performance of the composite is evaluated. SEM and TEM analyses show a structural alteration of the composites from Si nanoparticle-surrounded graphite nanosheets to Si nanoparticle-embedded graphite nanosheets by decreasing the size of Si nanoparticles from 250 nm to 40 nm. The composites with finer Si nanoparticles provide an effective nanostructure containing encapsulated Si and free space. This structure facilitates the indirect exposure of Si to electrolyte and Si expansion during cycling, which leads to a stable solid–electrolyte interphase and elevated conductivity. An enhanced rate capability was obtained for the 40 nm Si nanoparticle–graphite nanosheet composite, delivering a specific capacity of 276 mAh g−1 at a current density of 1 C after 1000 cycles and a rate capacity of 205 mAh g−1 at 8 C.

Strain and optical characteristics analyses of three-dimentional self-ordered multilayered SiGe nanodots by photoluminescence and Raman spectroscopy

The strain state, optical properties, and band structure of the self-ordered multilayered silicon-germanium (SiGe) nanodots, which are staggered and dot-on-dot alignment and embedded by Si spacer, were evaluated by Raman spectroscopy and low-temperature photoluminescence (PL). These results suggest that the compressive strain applied to the staggered nanodots is smaller than that of the dot-on-dot nanodots, which contributes to the shrinking of the bandgap of the staggered nanodots. Strong PL intensity was observed from the nanodots compared to the single crystalline bulk SiGe due to the carrier confinement and high crystal quality of the nanodots. The stack-controlled nanodots showed a redshift of the PL peaks compared to the bulk SiGe and the effect of strain induced in SiGe nanodots might not be enough to explain this phenomenon. The cause of the redshift was clarified by considering the hetero band structure of the nanodots and the tensile strained spacer.

Effect of heat treatment on microstructural characteristics and mechanical properties of hypereutectic B390 aluminium alloys modified by phosphorus and bismuth

This study investigated the effects of solution treatment and aging processes on microstructural characteristics and mechanical properties of hypereutectic B390 Al-Si Alloy modified by phosphorus (P) and bismuth (Bi). The morphology evolution of primary Si, eutectic Si, intermetallic compounds, and secondary dendrite arm spacing (SDAS) of both initial and modified alloys were investigated in as-cast and heat-treated conditions. It was found that the combination of 0.1 wt%P and 0.5 wt%Bi in B390 alloy resulted in a remarkable modification and refinement of primary Si, eutectic Si, and SDAS compared to initial specimens in the as-cast condition. After heat treating, the fine lamellar eutectic Si and intermetallic compounds underwent a higher degree of fragmentation and coarsening, appearing as small round particles, leading to a more pronounced formation of a supersaturated solid solution. Additionally, the peak aging time of the B390-P-Bi alloy was only 7 h, 2 h less than the initial specimen. After T6 treatment and addition of both 0.1 wt%P and 0.5 wt%Bi, the B390-P-Bi alloy exhibited a smaller rounded Mg3Bi2 phase at peak aging with a 62.07% size reduction compared to the B390-Bi alloy. The results show that adding P with Bi enhanced hardness, ultimate tensile strength (UTS) and elongation because of refinement of the SDAS, primary Si and eutectic Si. These refinements played a crucial role in increasing tensile strength by more effective strengthening through precipitation and grain size reduction. The B390-P-Bi alloy is a promising choice for applications requiring a balance between strength and ductility.

Si-TiSi2 clusters eutectic nanoparticles as high initial coulombic efficiency anodes for lithium-ion batteries

The poor initial coulombic efficiency of silicon-based anode electrode is a major challenge hindering its practical application. In the initial cycle, the excessive consumption of lithium ions leads to significant reduction in energy density. Here, we propose a unique structure of eutectic nanoparticles containing TiSi2 clusters in the main phase of Si. The interaction between TiSi2 cluster and Si increases the lattice spacing of Si, reduces the lithium-ion insertion resistance and improves the initial coulombic efficiency of the electrode. The Si-TiSi2 clusters eutectic nanoparticles provide a high initial coulombic efficiency of 89.7% at 0.5 A g−1. This work provides a new enlightenment for high initial coulombic efficiency Si-based anodes.

3-Dimensional Self-Ordered Multilayered Ge Nanodots on SiGe

Multilayered Ge nanodots have drawn much attention due to their potential applications in optoelectronics, such as photodetectors and lasers. Many groups studied multilayered Ge nanodots with Si spacers on Si(001) grown by Stranski-Krastanov (SK) growth mode and vertically aligned by local tensile strain induced by buried Ge nanodots (1-2). However, to avoid plastic relaxation caused by a 4.2% lattice mismatch between Si and Ge, thick nanodots and/or large layer numbers are challenging. Additionally, laterally-aligned Ge nanodots without pre-structuring have not been reported. In this study, we demonstrate 3-dimensional (3D) self-ordered Ge nanodots on SiGe virtual substrate (VS) by SiGe/Ge cyclic epitaxial growth and show the effects of fabrication parameters.The 3D self-ordered Ge nanodots were fabricated by reduced-pressure chemical vapor deposition. A Si0.4Ge0.6 VS with step-graded buffer deposited on Si(001) wafer was used. This VS was post-annealed at 1000°C, followed by chemical-mechanical polishing. After HF dip, the substrate was baked at 850°C in H2, then cooled down to 550°C for epitaxial growth. A 52-82 nm thick Si0.48Ge0.52 layer was deposited using a H2-SiH4-GeH4 gas mixture, then a self-assembled Ge nanodots layer via SK growth mechanism was deposited with 7.5-15.0 nm Ge coverage using a H2-GeH4 gas mixture. This Si0.48Ge0.52/Ge deposition cycle was repeated 5 to 20 times to fabricate the 3D self-ordered Ge nanodot stack. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the morphology and alignment of the Ge nanodots. The facets of the Ge nanodots were studied by analyzing the cross-section cuts of AFM images and confirmed by scanning transmission electron microscopy (STEM). Nano-beam diffraction (NBD) was used to study strain in the superlattice.Fig. 1(a-c) show the AFM images of the 5-cycle superlattices with Ge coverage 7.5 to 15.0 nm. In fig. 1(a), we can see mainly two types of nanodots, diamond (32%) and dome (68%). The height of the diamond-like nanodots is 9 nm with a standard deviation (s) of 2.6 nm while that of the dome-like nanodots is 23 nm with s=1.8 nm. With increasing Ge coverage, dome-like nanodots dominate (fig. 1(b)) and some nanodots merge with the adjacent nanodots (fig. 1(c)). Since the dome-like nanodots show a lower s in height than the diamond-like nanodots do, engineering of self-ordering is more feasible with the dome-like nanodot.Fig. 2(a-b) show the angle view of SEM images of Ge nanodots on 5- and 20-cycle superlattice of 12.5 nm-Ge/52 nm-SiGe. The dome-like nanodots are dominant and laterally aligned. The alignment and the uniformity improve with the increasing cycle number of the superlattice. However, when the thickness of Si0.48Ge0.52 spacer increases, the lateral- and vertical alignment of nanodots become random, and the amount of diamond-like nanodots increases (not shown).To study facets of two types of nanodots, the tilt of each facet was calculated from the cross-section cuts of AFM images. Fig. 3(a) shows an AFM image of the dome-like nanodot. Fig. 3(b) shows the cross-section cuts of fig. 3(a) along <110>. These cross-section cuts are well-overlapped with the high-angle annular dark-field (HAADF) STEM image as shown in the background of fig. 3(b). Therefore, it is possible and reliable to estimate the facets from the cross-section cuts of our AFM images. By this method, we confirmed that the diamond-like nanodot is composed of {105} and the dome-like nanodot is composed of {113} and {159}. This is consistent with a study of Ge dot on Si substrate except for {159} facet (3). Instead of {159} facet, a relatively similar facet {3 15 23} was reported. This difference may result from the less compressive strain in our Ge nanodots because SiGe VS was used.To explain the vertical correlation of Ge nanodots, a HAADF STEM image and in- and out-of-plane strain distributions measured by NBD are shown in fig. 4(a-c). The nanodots are vertically aligned (fig. 4(a)). The SiGe on the nanodot shows a relatively higher lattice parameter along <110> (fig. 4(b)) and lower lattice parameter along <001> (fig. 4(c)) compared to that on Ge wetting layer, indicating tensile strain. This tensile strain area is the preferred position for Ge nanodot formation because of less lateral lattice mismatch. Consequently, the nanodots tend to grow above the buried nanodots.3D self-ordered multilayered Ge nanodots on SiGe VS were successfully fabricated, and the facets and the vertical correlation of Ge nanodots were studied. References P.S. Chen et al. Materials Science and Engineering B 108 (2004) 213-218K.L. Wang et al. Proceeding of the IEEE 95 (2007) 1866J.T. Robinson et al. Nanotechnology 20 (2009) 085708 Figure 1

Elucidating thermal conductivity mechanism of Al-9Si based alloys with trace transition elements (Mn, Cr, V)

The effects of trace transition elements (Mn, Cr, V) on the solidification microstructure and the thermal conductivity of Al-9Si based alloys have been investigated. It was found that the effect of Mn, Cr, V on the solidification behavior and solidification microstructure is mainly related to α-Al grains via affecting heterogeneous nucleation and growth restriction, while no significant effect on the solidification behavior and microstructure of eutectic Si was observed. Compared with factors including grain size of α-Al and morphology characteristics (e.g. size, spacing, and shape factor) of eutectic Si, the solid solubility in Al matrix is confirmed to be one of the most important factors affecting thermal conductivity. Due to the larger partition coefficient of V compared with Cr and Mn, the solid solubility and the lattice distortion of α-Al in Al-9Si-xV alloy were increased significantly, thus the thermal conductivity of alloy was decreased steeply from 171.7 to 152.3 W/m·K with the content of V increasing from 15 to 1010 ppm. A linearly fitted curve reflecting the relationship between the content of the transition elements and the thermal conductivity of alloys has been proposed, which can be used to optimize the alloy composition in subsequent remelting or recycling process. Furthermore, two thermal conductivity models (H-S model and Maxwell model) also have been used to evaluate the effect of Mn, Cr, V on thermal conductivity, in which the thermal conductivity of α-Al was corrected based on the measured solid solubility. In this way, the data correlation between the experimental and fitted value based on H-S model and Maxwell model is increased from 0.18 to 0.96. These obtained results could be helpful to precisely control the composition and further improve the thermal conductivity of Al-Si based alloy during the primary production and / or recycling process.

Effect of specimen processing for transmission electron microscopy on lattice spacing variation in Si specimens

Variation in the (220) lattice spacing of Si due to specimen processing for transmission electron microscopy (TEM) was experimentally evaluated by comparing the measured lattice spacings of crystalline specimens processed by crushing, mechanical polishing only, and combined mechanical and Ar ion polishing. Although distinct variation in the (220) lattice spacing between the Si specimens processed by crushing and by mechanical polishing only is not observed, the (220) lattice spacing of specimens prepared by combined mechanical and Ar ion polishing imply increasing tendency with increasing Ar ion beam irradiation time. Moreover, the (220) lattice spacing measured from the outermost region of the specimen edge tends to be approximately 3% to 5% larger than that measured from the inner region, irrespective of the processing method. These results demonstrate that differences in the processing conditions of Si specimens and in the measurement location of the Si lattice spacing can be major component of the measurement uncertainty in sub-nanometer metrology using TEM with magnification calibration by the Si lattice spacing. When attempting to apply the lattice spacing of Si as a reference with traceability to the International System of Units for TEM magnification calibration in sub-nanometer metrology, the results suggest that the effect of specimen processing on variation in the lattice spacing is not negligible.

Realization of a cost-effective thermochromic solar absorber with a high emittance change based on VO2 and an infrared transparent intermediate layer

In this study, we present a thermochromic solar absorber coating that reaches a high thermal emittance change by using a thin, optically switching VOx film located on an infrared transparent interlayer (spacer) of Si or Ge with an optical thickness of λ/4 (for λ = 7 μm). Using this so-called “lambda/4-concept,” temperature-dependent reflection measurements in the spectral range between 2500 and 50 000 nm from an absorber with a 450 nm Si spacer and a VOx film oxidized from a 30-nm-thick V display an overall increase in emittance from ε(25 °C) = 12.2% to ε(150 °C) = 55.1%, resulting in a change of Δε = 42.9%. In addition, using an absorber with a 400 nm Ge spacer in combination with a VOx film oxidized from 17.5-nm-thick V, an increase in emittance from ε(25 °C) = 8.2% to ε(150 °C) = 49.2% with a change of Δε = 41.0% was achieved. In addition, the optical properties of Ge and Si thin films over a wide spectral range of 250–38 000 nm were determined using spectroscopic ellipsometry. Using this optical data and a simple optical model of the VOx film, reflectance simulations could be performed by using the ellipsometry analysis software WVASE©. It was shown by x-ray diffraction measurements that the optically switching VOx films oxidized from V in a belt furnace consist of the VO2 and the V2O5 phases. Using scanning electron microscopy images, the surface morphology of optically switching VOx films was compared with over-oxidized VOx films. A correlation between the surface morphology and the crystalline phases was revealed and applied to search for the optimal furnace parameters. This method could significantly reduce the time cost to achieve optically switching VOx coatings using an oxidation process.

Photoluminescence enhancement by deterministically site-controlled, vertically stacked SiGe quantum dots

The Si/SiGe heterosystem would be ideally suited for the realization of complementary metal-oxide-semiconductor (CMOS)-compatible integrated light sources, but the indirect band gap, exacerbated by a type-II band offset, makes it challenging to achieve efficient light emission. We address this problem by strain engineering in ordered arrays of vertically close-stacked SiGe quantum dot (QD) pairs. The strain induced by the respective lower QD creates a preferential nucleation site for the upper one and strains the upper QD as well as the Si cap above it. Electrons are confined in the strain pockets in the Si cap, which leads to an enhanced wave function overlap with the heavy holes near the upper QD’s apex. With a thickness of the Si spacer between the stacked QDs below 5 nm, we separated the functions of the two QDs: The role of the lower one is that of a pure stressor, whereas only the upper QD facilitates radiative recombination of QD-bound excitons. We report on the design and strain engineering of the QD pairs via strain-dependent Schrödinger-Poisson simulations, their implementation by molecular beam epitaxy, and a comprehensive study of their structural and optical properties in comparison with those of single-layer SiGe QD arrays. We find that the double QD arrangement shifts the thermal quenching of the photoluminescence signal at higher temperatures. Moreover, detrimental light emission from the QD-related wetting layers is suppressed in the double-QD configuration.

Capacitance Scaling in In0.71Ga0.29As/InP MOSFETs With Self-Aligned a:Si Spacers

In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.71</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.29</sub> As/InP (12/2 nm) quantum well MOSFETs using sacrificial amorphous silicon (a:Si) spacers to achieve low parasitic capacitance are fabricated. Radio frequency characterization of 73 devices is used to study the intrinsic and extrinsic capacitances. The total gate intrinsic parasitic capacitance of 0.55 fF/ μm is achieved with an intrinsic gate capacitance of 0.39 μF/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The various parasitic capacitances are modeled using finite element electrostatic simulations, and semi-analytical expressions are provided. A device with a gate length L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> =80 nm has SS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> =168 mV/dec, dc transconductance g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>m,e</i></sub> = 1.0 mS/ μm at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> =0.5 V, and exhibits a peak cutoff frequency <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>T</i></sub> of 243 GHz, and a maximum oscillation frequency <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>max</i></sub> of 147 GHz at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> =0.25 V, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = 1 V.

Synthesis of superlattice heterostructure of germanium quantum dots in silicon spacer layers and its application in photovoltaic solar cells

Superlattice heterostructure of dislocation-free Ge quantum dots buried in Si spacer layers has been synthesized using ultra high vacuum-chemical vapor deposition technique. The heterostructure has been used as active layer in Si solar cells based upon vertical p-i-n junction configuration. Experiment shows that such solar cells exhibit strong repose to infrared radiation in contrast with the negligible response of solar cells with active layers of pure Si having the same thickness. A solar cell with 300 nm thick superlattice is shown to have thermodynamic efficiency of about 11.2%. This is correlated with the rather high quality of the heterostructure used in the present study as judged from photoluminescence spectroscopy and electron microscopy as well as strain and dark current measurements. It is concluded that such type of heterostructure may provide a pathway to develop efficient and cost effective solar cells. • Superlattice heterostructure of Ge quantum dots on Si has been synthesized. • Various techniques have been used to characterize the nanostructures. • The heterostructure has been used as active layer in Si (p-i-n)-type solar cells. • The thermodynamic efficiency of respective solar cell is determined.

Neuro-fuzzy based predictive model for cutting force in CNC turning process of Al\u2013Si\u2013Cu cast alloy using modifier elements

This study investigated the predicted cutting force model of a turning operation for Al–Si–Cu cast alloy modified with modifiers based on adaptive neuro-fuzzy inference system (ANFIS) approach. Feed rate, cutting speed and Silicon spacing were considered as the input parameters. A series of turning experiments were conducted at various feed rates and cutting speeds. The prediction result showed that the ANFIS model successfully predicted the cutting force value in terms of cutting speed, feed rate and Si spacing. A mathematical model was proposed to describe the cutting force changes during the machining of Al–Si–Cu cast alloy. Moreover, the addition of Bismuth into the base alloy decreased the cutting force compared to other refinement elements.

Wet and Siconi® Surface Preparation Sequences for SiGe Epitaxial Regrowth

The efficiency of the “Siconi®” process, which is based on the use of a NH3 / NF3 remote plasma and anneals at temperatures less than 180°C, was recently assessed on SiGe surfaces. Siconi® removed GeO2 and SiO2, but was less efficient on GeOx. The presence of residual GeOx after a Siconi® process was minimized with the use, beforehand, of a wet oxidation. It consisted on dips in Standard Cleaning 1 (SC1) solutions made of NH4OH, H2O2 and H2O or in water with a few ppm of Ozone. A SiO2 - rich oxide was generated, then (P.E. Raynal et al, Microelec. Eng. 187-188, 84 (2018)). In this study, we elaborate on those findings by comparing the performances of (i) wet, (ii) Siconi® and (iii) “wet - Siconi®” sequences on re-epitaxy on SiGe surfaces. Those sequences were evaluated on 15 nm thick Si0.6Ge0.4 layers with different Queue-times (15 min and 8 hours) between wet and Siconi® treatments. Without any air break, the Siconi® process was followed by a low temperature H2 bake (at 20 Torr and a temperature less than 700°C) and a re-epitaxy at 600°C of 15 to 17 nm of Si0.6Ge0.4. Secondary Ions Mass Spectrometry (Figure 1) showed that a “chemical oxide - Siconi” sequence yielded a definite reduction of the interfacial oxygen concentration. Even with 8 hours of Q-time, concentrations were very low with these sequences. They were (i) more than one order of magnitude lower than with a regular “HF-HCl” (i.e. a “HF-Last”) wet cleaning and (ii) several times lower than with Siconi® only. Atomic Force Microscopy (AFM) images of the surfaces (Figure 2) of ~ 30 nm thick Si0.6Ge0.4 layers (after epitaxial regrowth) showed the presence of numerous islands after the use of “chemical oxide - Siconi®” surface preparations. Those defects were associated with poor crystalline quality layers. This was surprising. Indeed, such sequences yielded a superior oxide removal efficiency (Fig. 1) and smooth and defects-free SiGe surfaces just after their use (i.e. without any re-growth). Other epitaxial regrowth schemes using solely “HF/HCl” or Siconi® treatments did not result in such defects. The use of “chemical oxide - Siconi®” sequences must have resulted, after oxide removal, in SiGe surfaces richer in germanium than the “bulk” concentration (40%). They were then more sensitive to 2D-3D transitions during the H2 bake than “regular” SiGe 40% surfaces. Ge-rich islands were thus formed during the H2 bake that followed the “chemical oxide-Siconi®” sequence, destabilizing the epitaxial re-growth. In order to prevent surface islanding after "chemical oxide-Siconi®" sequences, we have then evaluated the interest of adding dichlorosilane (DCS) to H2 during the temperature stabilization (just after loading inside the epitaxy chamber), the temperature ramping-up then the H2 bake itself. Our aim was to encapsulate the extreme surface with a few atomic planes of Si in order to prevent 3D transitions. The reason why DCS was used was the following: with this chlorinated precursor, the homo-epitaxial Si growth rate decreased exponentially with the temperature (Ea > 2eV, typically), with a lower “boundary” of 650°C, at which it was only 5 Å/min. When sent on a SiGe surface at T < 650°C, growth with DCS essentially stopped as soon as a switch over to a pure Si surface occurred, resulting in really thin Si caps. The H2 annealing prior to SiGe epitaxy was divided into three steps:1) low temperature stabilization of the wafer (after loading) ;2) ramping-up to the bake temperature ;3) low thermal budget H2 bake (2 min. at T < 700°C).Three epitaxial regrowth schemes, with "HF/HCl-SC1-Siconi®" surface preparations then injections of dichlorosilane during step 1) only, steps 1) and 2) or steps 1), 2) and 3), were evaluated. As shown by AFM (Figure 3), injection of DCS during the various annealing phases drastically reduced the density of 3D defects or suppressed them. The Si spacer thickness between the 15 nm thick Si0.6Ge0.4 layer and the 17 nm of re-epitaxy after the addition of DCS during steps 1) and 2) was 2 nm only (Figure 4). This is acceptable from an integration point of view, especially in SiGe:B sources and drains grown on top of high hole mobility SiGe channels, as such stacks will be germano-salicided afterwards, anyway. Figure 1

Analysis for nonradiative recombination loss and radiation degradation of Si space solar cells

AbstractSilicon space solar cells are currently attracting attention again for their relatively low‐cost feature with sufficient performance, and they are expected to resume into the space market especially by short‐term mission spacecraft designers. In this paper, efficiency potential of crystalline Si space solar cells is analyzed by considering external radiative efficiency (ERE), voltage and fill factor losses. Crystalline Si space solar cells have efficiency potential of more than 26% by realizing ERE of 20% from about 0.2% and normalized resistance of less than 0.05 from around 0.15. Nonradiative recombination and resistance losses in Si space solar cells are also discussed. Radiation degradation of Si space solar cells is also analyzed. Advanced Si solar cells such as passivated emitter, hetero‐junction, and back contact solar cells are expected to use as space solar cells. Potential of advanced Si solar cells for space applications is discussed from point view of radiation degradation.

Photoluminescence of compact GeSi quantum dot groups with increased probability of finding an electron in Ge

The photoluminescence (PL) of the combined Ge/Si structures representing a combination of large (200–250 nm) GeSi disk-like quantum dots (nanodisks) and four-layered stacks of compact groups of smaller (30 nm) quantum dots grown in the strain field of nanodisks was studied. The multiple increase in the PL intensity was achieved by the variation of parameters of vertically aligned quantum dot groups. The experimental results were analyzed on the basis of calculations of energy spectra, electron and hole wave functions. It was found that the quantum dot arrangement in compact groups provides the effective electron localization in Δx,y-valleys with an almost equal probability of finding an electron in the Si spacer and Ge barrier. As a result, the main channels of radiative recombination in the structures under study correspond to spatially direct optical transitions.

Effects of solidification thermal parameters and Bi doping on silicon size, morphology and mechanical properties of Al-15wt.% Si-3.2wt.% Bi and Al-18wt.% Si-3.2wt.% Bi alloys

The use of bismuth (Bi) in aluminum (Al) industries is recognized as prime importance because of possible recycling as alloying element in special applications such as those enhancing machinability. The effects of Bi interaction on solidification progress as well as on the resulting microstructure of hypereutectic Al-Si alloys, i.e. the morphology and size of Si crystals are rare information. As such, the present research work investigates the characteristic parameters of eutectic and primary Si with addition of Bi for both ternary Al-15 wt.%Si- 3.2 wt.%Bi and Al-18 wt.%Si- 3.2 wt.%Bi alloys samples directionally solidified (DS) under various cooling rates. The alloy containing 15 wt.%Si showed that Bi acts in the suppression of the growth of primary Si crystals for samples solidified at cooling rates ranging from 2.0 to 31 K/s. For lower rates, a minor fraction of five-fold branched Si was recognized in transverse specimens. A mixture of globular-fibrous eutectic Si is shown to occur for eutectic cooling rates higher than 5 K/s. The Al-18 wt.%Si- 3.2 wt.%Bi alloy is characterized by a mixed structure of eutectic flaky Si and primary angular Si with intersecting spines at nearly 90º all over the entire range of experimental cooling rates (i.e., from 0.4 to 40 K/s). Despite the intricacy of the formed microstructures, it was possible to establish relationships between tensile properties and eutectic Si spacing in order to compare the properties between the tested alloys. Tensile strength and elongation decrease with increasing alloy Si content.